Homology model of the CB1 cannabinoid receptor: sites critical for nonclassical cannabinoid agonist interaction.

Association of cannabimimetic compounds such as cannabinoids, aminoalkylindoles (AAIs), and arachidonylethanolamide (anandamide) with the brain cannabinoid (CB(1)) receptor activates G-proteins and relays signals to regulate neuronal functions. A CB(1) receptor homology model was constructed using the published x-ray crystal structure of bovine rhodopsin (Palczewski et al., Science, 2000, Vol. 289, pp. 739-745) in the conformation most likely to represent the "high-affinity" state for agonist binding to G-protein coupled receptors (GPCRs). A molecular docking approach that combined Monte Carlo and molecular dynamics simulations was used to identify the putative binding conformations of nonclassical cannabinoid agonists, including AC-bicyclic CP47497 and CP55940, and ACD-tricyclic CP55244. Placement of these ligands was based upon the assumption of a critical hydrogen bond between the A-ring OH and the side chain N of Lys192 in transmembrane helix 3. We evaluated two alternative binding conformations, C3-in and C3-out, denoting the directionality of the ligand C3 side chain within the receptor with respect to the inside or the outside of the cell. Assuming both the C3-in or C3-out conformation, the calculated ligand-receptor binding energy (DeltaE(bind)) was correlated with the experimentally observed binding affinity (K(i)) for a series of nonclassical cannabinoid agonists. The C3-in conformation was marginally better than the alternative C3-out conformation in predicting the rank order of the tested nonclassical cannabinoid analogs. Adopting the C3-in conformation due to the greater number of receptor interactions with known pharmacophoric elements of the ligand, key residues were identified comprising the presumed hydrophobic pocket that interacts with the C3 side chain of cannabinoid agonists. Key hydrogen bonds would form between both K3.28(192) and E(258) and the A-ring OH, and between Q(261) and the C-ring C-12 hydroxypropyl. In summary, the present study represents one of the first attempts to construct a homology model of the CB(1) cannabinoid receptor based upon the published bovine rhodopsin x-ray crystal structure and to elucidate the putative ligand binding site for nonclassical cannabinoid agonists. We postulated sites of the CB(1) receptor critical for the ligand interaction, including the hydrophobic pocket interacting with the key pharmacophoric moiety, the C3 side chain. More work is needed to delineate between two alternative (and possibly other) binding conformations of the nonclassical cannabinoid ligands within the CB(1) receptor. The present study provides a consistent framework for further investigation of the CB(1) receptor-ligand interaction and for the study of CB(1) receptor activation. Copyright 2003 Wiley Periodicals, Inc.